Navigate to one of the restore screens, such as [Restore VM as Clone] or [File Level Restore] for instance, then choose [Add Restore Source]

You will then be prompted with a wizard for selecting the restore source, be it a Local Drive, Network Path, Altaro Offsite Server and Azure Cloud Storage Account

Follow the on-screen instructions and click [Finish] to complete

Once done, the new Restore Source will show up in the list of locations, so simply tick the box next to the Restore Source you added and click [Next]

You will then be shown a list of VM backups detected in the selected Restore Source. Selecting 1 VM will suffice, then click [Next]

You'll be presented with the option to choose the version you wish to restore, in this case we're only really interested in the [Decryption Key] field. So in this case, simply enter your key and you should get a green check mark is the entered key is correct.